1. Field of the Invention
The present invention relates to the control of orbiting spacecraft and more particularly to a method and means for reducing short term transients caused by roll momentum unloads.
2. Problem to be Solved
Earth-orbiting spacecraft, such as communications satellites, require orientation control to minimize excessive movements in pitch, yaw, and roll, that can effect their remaining in proper orbit, and that can interfere with their pointing in a proper direction to insure the reception of signals transmitted therefrom at receiving stations on the ground. Various systems are provided on the spacecraft to affect this control involving momentum wheels, thrusters, magnetic torquers, sensors for yaw, roll, and pitch, a digital integrating rate assembly (DIRA), nutation dampers, etc. For example, such control systems use thrusters to unload unwanted momentum for preventing orientation drift.
In dealing with short-term transients due to disturbances such as roll momentum unloads, currently the spacecraft is biased in roll angle, approximately 50% of the magnitude, in the opposite direction of the expected transient. A problem with this approach is that the angle pre-bias must be removed at some future point, which removal can cause a transient of its own. In addition, removal timing is always critical.
It would therefore be desirable to have a system for controlling spacecraft orientation that does not rely on changing or removing the angle pre-bias.
Objects
It is accordingly an object of the present invention to provide an automatic, on-board solution to the problem of dealing with short-term transients imposed on a spacecraft due to roll momentum unloads.
It is also an object of the invention to provide a method and means for spacecraft orientation control that will reduce short-term transients due to roll momentum unloads.
It is another object of the invention to provide a system for controlling spacecraft using the nutation compensator to reduce short-term transients and avoid changing or removing the angle pre-bias.
The present invention involves the provision of an automatic, on-board system that controls and reduces short-term transients in orbiting spacecraft, caused by disturbances such as roll momentum unloads, by manipulating the states of the spacecraft""s nutation compensator. The system is used to reset one of the states in the nutation compensator polynomial to a value in proportion to the magnitude of the expected roll momentum unload so as to develop a short-term transient that is equal and opposite in magnitude to that of the roll unload. The compensator will, of its own accord, automatically damp out the roll unload transient without biasing the spacecraft in roll. No angle bias need be changed or removed.